The Applicants' earlier PCT patent application WO2006/097767, describes an SCP camshaft in which the positions of the drive members are adjustable in order to compensate for significant manufacturing inaccuracies between the inner shaft and its associated group of cam lobes. FIGS. 1A to 1E in the accompanying drawings correspond to FIGS. 2A to 2E respectively of the latter publication, which is incorporated herein by reference. In these drawings:
FIG. 1A is a side view of an SCP camshaft,
FIG. 1B is a section along the line I-I in FIG. 1A,
FIG. 1C is a section along the line II-II in FIG. 1A,
FIG. 1D is a partially exploded perspective view of the camshaft of Figure A, and
FIG. 1E is a partially cut-away perspective view of the camshaft of FIG. 1A.
The SCP camshaft 10 is made up of an inner shaft 12 and an outer tube 14, the latter being supported in bearings 20. A first group of cams 16 is secured, for example by heat shrinking, for rotation with the outer tube 14 and a second group of cams 18 is secured for rotation with the inner shaft 12 by drive members 50 having the form of compound fastener each consisting of a nut 50a and a bolt 50b. 
The shank of the bolt 50b passes with clearance through a hole in the drive shaft 12, and the head of the bolt and the nut act as drive members and are a tight clearance or an interference fit in the cam lobe 18.
In order to transmit torque between the cam lobe 18 and the inner drive shaft 12, the bolt and the nut are clamped against flat surfaces 12a, 12b on opposite sides of the drive shaft 12. The timing of each cam lobe 18 is therefore dictated by the position of the flat surfaces on the drive shaft 12 and the angle of the connecting pin bore in the cam lobe 18. The arrangement is shown clearly in FIGS. 1C and 1E.
An important aspect of this design is that once the two parts 50a, 50b of the fastener have been clamped on to the drive shaft 12, there must be no movement of the parts when the camshaft is in operation, as this will result in the camshaft becoming tight to turn. It is clearly an advantage therefore to maximise the coefficient of friction between the flat surfaces 12a and 12b of the drive shaft 12 and the parts of the fastener serving as a drive member, as this will increase the torque that can be applied to the cam lobe before any relative movement will take place.